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Tue, 13 Jul 2021 05:48:56 +0000 From: Alexander Bulekov To: qemu-devel@nongnu.org Subject: [PULL 1/3] fuzz: fix sparse memory access in the DMA callback Date: Tue, 13 Jul 2021 01:48:34 -0400 Message-Id: <20210713054836.154421-2-alxndr@bu.edu> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20210713054836.154421-1-alxndr@bu.edu> References: <20210713054836.154421-1-alxndr@bu.edu> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MN2PR03CA0028.namprd03.prod.outlook.com (2603:10b6:208:23a::33) To SN6PR03MB3871.namprd03.prod.outlook.com (2603:10b6:805:6d::32) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from stormtrooper.vrmnet (72.74.210.193) by MN2PR03CA0028.namprd03.prod.outlook.com (2603:10b6:208:23a::33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4308.20 via Frontend Transport; Tue, 13 Jul 2021 05:48:55 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: c79a7481-c2f7-40b2-d188-08d945c1e747 X-MS-TrafficTypeDiagnostic: SN2PR03MB2285: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:7691; 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envelope-from=alxndr@bu.edu; helo=NAM11-BN8-obe.outbound.protection.outlook.com X-Spam_score_int: -8 X-Spam_score: -0.9 X-Spam_bar: / X-Spam_report: (-0.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HK_RANDOM_ENVFROM=0.001, HK_RANDOM_FROM=0.998, MSGID_FROM_MTA_HEADER=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , Darren Kenny , Alexander Bulekov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" The code mistakenly relied on address_space_translate to store the length remaining until the next memory-region. We care about this because when there is RAM or sparse-memory neighboring on an MMIO region, we should only write up to the border, to prevent inadvertently invoking MMIO handlers within the DMA callback. However address_space_translate_internal only stores the length until the end of the MemoryRegion if memory_region_is_ram(mr). Otherwise the *len is left unmodified. This caused some false-positive issues, where the fuzzer found a way to perform a nested MMIO write through a DMA callback on an [address, length] that started within sparse memory and spanned some device MMIO regions. To fix this, write to sparse memory in small chunks of memory_access_size (similar to the underlying address_space_write code), which will prevent accidentally hitting MMIO handlers through large writes. Signed-off-by: Alexander Bulekov Reviewed-by: Darren Kenny Reviewed-by: Philippe Mathieu-Daudé --- tests/qtest/fuzz/generic_fuzz.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c index 6c67522717..0ea47298b7 100644 --- a/tests/qtest/fuzz/generic_fuzz.c +++ b/tests/qtest/fuzz/generic_fuzz.c @@ -240,10 +240,17 @@ void fuzz_dma_read_cb(size_t addr, size_t len, MemoryRegion *mr) addr, &addr1, &l, true, MEMTXATTRS_UNSPECIFIED); - if (!(memory_region_is_ram(mr1) || - memory_region_is_romd(mr1)) && mr1 != sparse_mem_mr) { + /* + * If mr1 isn't RAM, address_space_translate doesn't update l. Use + * memory_access_size to identify the number of bytes that it is safe + * to write without accidentally writing to another MemoryRegion. + */ + if (!memory_region_is_ram(mr1)) { l = memory_access_size(mr1, l, addr1); - } else { + } + if (memory_region_is_ram(mr1) || + memory_region_is_romd(mr1) || + mr1 == sparse_mem_mr) { /* ROM/RAM case */ if (qtest_log_enabled) { /* -- 2.28.0